Certificate pop-up error on outlook

Hi,
We have a client that we migrated from sbs 2003 with exchange 2003 to sbs 2011 with exchange 2010. We have mix workstation with XP Pro sp3 x32 and Win7 pro sp1 x64. Some computers have Outlook 2003, 2007 and 2010. We have TLS configured to communicate securely to a partner which is critical in this business with SSL Certificate that we installed on the server.

After the migration, we noticed that every time user opens their outlook, they get this pop-up of Security Alert.

Green check: The security certificate is from a trusted certifying authority.
Green check: The security certificate date is valid.
Red X: The name on the security certificate is invalid or does not match the name of the site.

I click yes to proceed and outlook communicates. We check the certificate which is our valid one that we bought from the domain hosting. OWA is working fine as it should internally and externally. We had tested in environment with similar setup and didn’t come across with this issue. Normally this certificate should not pop-up at all in the office since outlook should be connecting directly to the exchange. After doing some research, I noticed that “proxy over http” was enabled on 1 computer and disabled on another computer so far. I have not check on all computers.

So, what is causing this pop-up to occur since one week after the migration?

The link gave us the right direction and there was some setting we had to adjust. We have the Signed Certificate from CA not a self-signed. Also, we had to update on our Cloud email service to work properly with SBS 2011 with Exchange 2010.